The present invention relates to magnetic resonance imaging or xe2x80x9cMRI.xe2x80x9d
MRI is widely used in medical and other arts to obtain images of a subject such as a medical patient. The patient""s body is placed within a subject receiving space of a primary field magnet and exposed to a strong, substantially constant primary magnetic field. The atomic nuclei spin around axes aligned with the magnetic field. Powerful radio frequency xe2x80x9cRFxe2x80x9d signals are broadcast into the subject receiving space to excite atomic nuclei within the patient""s body into a resonance state in which the spinning nuclei generate minuscule RF signals, referred to herein as magnetic resonance signals. By applying magnetic field gradients so that the magnitude of the magnetic field varies with location inside the subject-receiving space characteristics of the magnetic resonance signals from different locations within the region, such as the frequency and phase of the signals, can be made to vary in a predictable manner depending upon position within the region. Thus, the magnetic resonance signals are xe2x80x9cspatially encodedxe2x80x9d so that it is possible to distinguish between signals from different parts of the region. After repeating this procedure with various different gradients, it is possible to derive a map showing the intensity or other characteristics of the magnetic resonance signals versus position within the excited region. Because these characteristics vary with concentration of different chemical substances and other characteristics of the tissue within the subject""s body, different tissues provide different magnetic resonance signal characteristics. When the map of the magnetic resonance signal characteristics is displayed in a visual format, such as on screen or on a printed image, the map forms a visible picture of structures within the patient""s body.
MRI provides unique imaging capabilities which are not attainable in any other imaging method. For example, MRI can provide vivid, detailed images of soft tissue abnormal tissues such as tumors, and other structures which cannot be seen readily in X-ray images. Moreover, MRI operates without exposing the patient to ionizing radiation such as X-rays. For these and other reasons, MRI is widely utilized in medicine.
Some of the primary field magnets utilized heretofore have imposed severe physical constraints on the patient and on medical personnel attending to the patient during the MRI procedure. For example, conventional solenoidal primary field magnets use a series of circular super-conducting coils spaced apart from one another along an axis. These magnets provide a small, tubular subject-receiving space enclosed within the solenoids. A patient to be imaged must slide into the tubular space. The experience is highly claustrophobic. Some obese or pregnant patients often cannot fit inside the patient-receiving space. Moreover, it is essentially impossible for a physician to reach those regions of the patient disposed inside the subject receiving space.
Attempts have been made heretofore to create xe2x80x9copenxe2x80x9d MRI primary field magnets using ferromagnetic frames. Although these designs provide somewhat better access to the patient for diagnostic scanning, and a somewhat less claustrophobic experience for the patient, they are less than optimal for surgical intervention. For example, these designs provide limited access of physicians and surgeons to the patient. Additionally, the designs have difficulty providing a highly uniform field with pole dimensions desirable for surgery.
As described, for example, in commonly assigned U.S. Pat. No. 4,707,663, other primary field magnets utilize ferromagnetic frames to route and concentrate magnetic flux into the patient receiving space. Primary field magnets using such a ferromagnetic frame can employ permanent magnets, resistive electromagnetic coils, or super-conducting coils having a relatively low number of ampere terms while still providing a high field strength in the patient-receiving space. Moreover, such magnet assemblies can provide excellent field uniformity. Ferromagnetic frame magnets in accordance with the ""663 patent also provide a less claustrophobic, more accessible subject receiving space.
As disclosed in co-pending, commonly assigned U.S. patent application Ser. No. 07/952,810 filed Sep. 28, 1992, the disclosure of which is hereby incorporated by reference herein, a ferromagnetic magnetic frame may include a pair of plate-like pole supports spaced apart from one another and supported above one another by a set of columns. In preferred magnets according to ""810 application, the frame defines a polar axis passing through the space between the plates. Preferably, ferromagnetic poles project from the pole supports adjacent the polar axis, so that the poles define a subject receiving spacing at a medial plane, midway between the plates. The columns have unique shapes such that, in preferred embodiments, the columns flare outwardly in the radial direction, away from the polar axis adjacent the medial plane. The dimensions of each column in the circumferential direction, around the polar axis desirably taper so that the circumferential dimension of each column is at a minimum in a region adjacent the medial plane. As described in further detail in the ""810 application, magnets with ferromagnetic frames in accordance with preferred embodiments of the invention taught therein can provide a unique combination of accessibility and a large, aesthetically pleasing and non-claustrophobic patient-receiving space and can also provide high field strength without resort to super-conducting coils. Even higher field strengths can be provided where superconducting coils are used. Magnets according to preferred embodiments taught in the ""810 application thus provide an elegant solution to the problems of claustrophobia, lack of access and limitations on field strength and uniformity posed by prior designs. Surgical operations and other medical procedures can be performed readily on a patient while the patient is disposed inside the patient-receiving space of preferred magnets according the ""810 application. The ability to perform surgical operations while the patient is disposed inside the patient-receiving space allows the physician to treat the patient under direct guidance of a MRI image acquired during the procedure itself. For example, as the surgeon advances a probe into the body to treat a lesion, the surgeon can see the probe and the lesion in the MRI image.
However, even with this enhanced design, the patient still perceives the MRI procedure as involving placement of his or her body into the interior of a machine. Moreover, the physician treating the patient still perceives that he or she must stand outside of the apparatus and reach into the apparatus to gain access to the patient. Accordingly, even further improvement in primary field magnet structures for MRI apparatus would be desirable.
One aspect of the present invention provides a magnet for magnetic resonance imaging apparatus which includes a frame. The frame desirably incorporates a pair of opposed ferromagnetic pole supports spaced apart from one another and a pair of ferromagnetic poles connected to the pole support. The poles project from the pole supports toward one another along a polar axis. The poles have distal ends remote from the pole supports. The distal ends confront one another and are spaced apart from one another by a gap distance so as to define a subject-receiving gap between the poles. The frame further includes one or more connecting elements extending between the pole supports. The connecting elements are spaced apart from the poles in a direction or directions transverse to the polar axis. The magnet further includes a source of magnetic flux adapted to direct flux through the frame so that the flux passes between the distal ends of the poles through the gap and returns through the pole supports and the connecting elements.
Most preferably, the magnet defines a working space alongside of the poles, between the pole supports and the poles and between the connecting elements sufficient to accommodate one or more adult human attendants. Thus, an attendant can be positioned inside the working space, within the magnet itself and can have access to a patient disposed in the gap between the poles. The working space desirably is about six feet or more high and about two feet or more wide, so that the attendant can work in a standing position. Most preferably, the working space extends entirely around the poles, and is unobstructed by any feature of the magnet itself. The magnet desirably includes a plurality of enclosing structures including walls, a floor and a ceiling which cooperatively define a room. The poles extend into the room, but the remainder of the frame desirably is at or outside the exterior of the room. For example, where the pole supports are spaced vertically apart from one another and the polar axis extends vertically, the poles project into the room from the floor and ceiling. Thus, the patient experiences entry into the MRI magnet as entry into a normal room with some structures extending from the floor and ceiling. Stated another way, the elements such as the connecting elements and pole supports are so far away from the patient that they do not create any feeling of claustrophobia. Because the physician or other attendant is inside the room and inside the space enclosed by the pole supports and connecting elements, these elements do not impede access by the physician to the patient at all. The connecting elements may be in the form of plates constituting one or more walls of the room as well as providing the pole supports which may be formed as further plates constituting the floor and ceiling of the room. The enclosing structure may further include concealment structure which conceals those parts of the frame constituting the walls from view from within the room. For example, the interior surfaces of the plates may be covered with conventional wall, floor and ceiling coverings. This contributes to the patient""s belief that he or she is inside a normal room.
Because the pole supports and connecting elements are disposed outside of the area occupied by the patient and attendant, these elements can be of essentially unlimited size. Essentially any amount of ferromagnetic material can be used to provide a low reluctance flux return path and to perform uniform distribution of flux passing to the poles. Magnets in accordance with preferred aspects of the present invention thus can provide a highly concentrated, strong magnetic field in the subject receiving gap. Magnets according to this aspect of the invention can utilize permanent magnets, super-conducting coil or, resistive electromagnetic coils as the source of electromagnetic flux. In a particularly preferred arrangement, a coil such as a resistive electromagnetic coil encircles each pole. Thus, the working space extends around the poles between the coils. Where the polar axis extends vertically, the working space desirably extends above one coil and below the other coil.
The gap distance between the distal ends of the pole preferably is about two feet or more and most preferably at least about three feet so as to provide an extraordinarily open, non-claustrophobic space for the patient and excellent access for the physician. In a particularly preferred arrangement, the gap distance is between about 3 feet and about 4 feet. The distal ends of the poles may be either circular or non-circular. Where the distal ends of the poles are circular, the ratio between the diameter of each pole distal end and the gap distance between the poles is desirably less than about 2 to 1. Where the distal ends of the poles are non-circular, the ratio between the longest dimension of each pole surface and the gap distance is also desirably less than about 2 to 1, and the ratio of the shortest dimension of each pole surface to the gap distance desirably is about 1.5:1 or less. The magnet desirably incorporates features to further enhance field uniformity in the patient receiving gap. Where coils are employed as the source of magnetic flux, each coil encircles the associated pole. Also, the magnet desirably includes shimming features such as shim rings, slots or other elements defining magnetic flux paths having different reluctances at different distances from the polar axis. To further promote field uniformity, each pole may include a pole tip defining a distal end of the pole and a pole stem extending from the proximal end of the pole to the pole tip. The flux source is arranged to direct the flux in a forward direction through each pole. The magnet may include stem bucking magnets surrounding the pole stem. The stem bucking magnets desirably provide flux directed in a reversed direction opposite to the forward direction. This tends to minimize leakage of flux from the pole stems to the connecting elements. The relatively large spacing between the poles and the connecting elements in radial directions transverse to the polar axis helps to minimize flux leakage from the poles, so that a very large portion of the flux tends to pass between the poles. This further promotes flux uniformity and a strong field in the subject receiving gap.
In a particularly preferred arrangement, the vertical connecting elements are disposed at least about 7 feet from the polar axis. Thus, a typical human patient can be positioned with the long axis of his or her body extending in any desired radial direction and with any portion of his or her body at the polar axis. For example, if the patient""s head is positioned at the polar axis, as where procedures or imaging are to be performed on the head, the patient""s feet can point in any direction. In one arrangement, the connecting elements include a pair of connecting elements such as a pair of opposed, heavy, plate-like walls disposed at least about 14 feet apart from one another and defining two opposite ends of a room. In other embodiments, the polar axis may extend horizontally, and the pole supports may extend along walls of the room defined by the magnet frame.